Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures

Observations of the varying orientations of comet tails led to the suggestion of the existence of the solar wind--a continuous outflow of ionized material from the Sun. It is now well established that gas from comets is ionized by several processes and joins the solar wind, forming an ion (plasma) tail that points away from the Sun. The plasma environments of three comets have been measured in situ, but only in the upstream direction or less than 8,000 km downstream of the nucleus. Here we report a fortuitous crossing by a spacecraft of the plasma tail of comet Hyakutake (C/1996 B2), at a distance of more than 3.8 astronomical units (550 million kilometres) from its nucleus. This surpasses the tail length of 2 AU determined for the Great March Comet of 1843 (C/1843 D1). Our measurements reveal that, at this distance, the tail of comet Hyakutake was a structured entity at least 7 million kilometres in diameter.     

（Ⅲ）m=0类二次系统的极限环问题，（Ⅰ）

彗星的空间探测是当代前沿课题．本文首先概述彗星的主要研究结果．彗星的本体是不大的‘脏雪球’彗核；随着接近太阳，其表面冰升华并带出尘埃而形成彗发，发生复杂的物理-化学过程；太阳辐射压和太阳风推斥彗发的尘埃和离子而形成彗尾；彗星尘散失在轨道附近而成为流星群．虽然地面观测到彗发和彗尾的丰富资料，但对彗核却了解甚少，需要发射飞船进行空间探测；接着，从探测彗核的真实性质、采集彗星样品分析成分、探索彗星活动和机制、彗星的有机物与生命起源、彗星-地球关系、太阳系起源和演化线索几方面阐述彗星的空间探测的科学意义；然后，综述彗星空间探测的一些计划进展情况，总结彗星空间探测的重要成果；最后，讨论未来的彗星空间探测的科学目标，包括高轨卫星采集彗星尘、搜寻近地小彗星、搜寻未知彗星、发射更优秀飞船探测不同彗星．     

Identification of two sources of carbon monoxide in comet Hale-Bopp.

The composition of ices in comets may reflect that of the molecular cloud in which the Sun formed, or it may show evidence of chemical processing in the pre-planetary accretion disk around the proto-Sun. As carbon monoxide (CO) is ubiquitous in molecular clouds, its abundance with respect to water could help to determine the degree to which pre-cometary material was processed, although variations in CO abundance may also be influenced by the distance from the Sun at which comets formed. Observations have not hitherto provided an unambiguous measure of CO in the cometary ice (native CO). Evidence for an extended source of CO associated with comet Halley was provided by the Giotto spacecraft, but alternative interpretations exist. Here we report observations of comet Hale-Bopp which show that about half of the CO in the comet comes directly from ice stored in the nucleus. The abundance of this CO with respect to water (12 per cent) is smaller than in quiescent regions of molecular clouds, but is consistent with that measured in proto-stellar envelopes, suggesting that the ices underwent some processing before their inclusion into Hale-Bopp. The remaining CO arises in the coma, probably through thermal destruction of more complex molecules.     

单细胞凝胶电泳技术在DNA损伤和细胞凋亡检测中的应用

用不同质量浓度的丝裂霉素C(MMC)、卡铂和5-氟尿嘧啶(5- Fu)处理Hela细胞24h,利用单细胞凝胶电泳( SCGE)技术分析这些药物对细胞DNA的损伤情况,结果发现:各药物处理组细胞都出现“彗星”,且随着各药物浓度的增大,彗星率升高,彗尾的荧光强度和长度也随之增大;各剂量组间存在良好的剂量-效应关系,差异显著...     

A large dust/ice ratio in the nucleus of comet 9P/Tempel 1

Comets spend most of their life in a low-temperature environment far from the Sun. They are therefore relatively unprocessed and maintain information about the formation conditions of the planetary system, but the structure and composition of their nuclei are poorly understood. Although in situ and remote measurements have derived the global properties of some cometary nuclei, little is known about their interiors. The Deep Impact mission shot a projectile into comet 9P/Tempel 1 in order to investigate its interior. Here we report the water vapour content (1.5 10(32) water molecules or 4.5 10(6) kg) and the cross-section of the dust (330 km2 assuming an albedo of 0.1) created by the impact. The corresponding dust/ice mass ratio is probably larger than one, suggesting that comets are 'icy dirtballs' rather than 'dirty snowballs' as commonly believed. High dust velocities (between 110 m s(-1) and 300 m s(-1)) imply acceleration in the comet's coma, probably by water molecules sublimated by solar radiation. We did not find evidence of enhanced activity of 9P/Tempel 1 in the days after the impact, suggesting that in general impacts of meteoroids are not the cause of cometary outbursts.     

Discovery of a comet by its Lyman-alpha emission

Several searches for near-Earth objects have recently been initiated, as a result of increased awareness of the hazard of impacts on the Earth. These programs mainly search for asteroids, so amateur astronomers can still contribute to the discovery of comets, especially out of the orbital plane of the Solar System. An ideal way to search for comets would be to use a spaceborne instrument capable of imaging the whole sky on a daily basis in a systematic and repeatable way. Such an instrument already exists on the solar observatory SOHO; it operates at the Lyman-alpha wavelength of neutral hydrogen, which is the main component of the emission cloud of a comet. Here we report the discovery, using archival data from this satellite, of a hitherto unnoticed comet which reached a perihelion of 1.546 a.u. on 26 June 1997. We derive the water production rate of the comet as a function of time and find that it increases after perihelion, like that of comet Halley.     

Compositional homogeneity in the fragmented comet 73P/Schwassmann-Wachmann 3

The remarkable compositional diversity of volatile ices within comets can plausibly be attributed to several factors, including differences in the chemical, thermal and radiation environments in comet-forming regions, chemical evolution during their long storage in reservoirs far from the Sun, and thermal processing by the Sun after removal from these reservoirs. To determine the relevance of these factors, measurements of the chemistry as a function of depth in cometary nuclei are critical. Fragmenting comets expose formerly buried material, but observational constraints have in the past limited the ability to assess the importance of formative conditions and the effects of evolutionary processes on measured composition. Here we report the chemical composition of two distinct fragments of 73P/Schwassmann-Wachmann 3. The fragments are remarkably similar in composition, in marked contrast to the chemical diversity within the overall comet population and contrary to the expectation that short-period comets should show strong compositional variation with depth in the nucleus owing to evolutionary processing from numerous close passages to the Sun. Comet 73P/Schwassmann-Wachmann 3 is also depleted in the most volatile ices compared to other comets, suggesting that the depleted carbon-chain chemistry seen in some comets from the Kuiper belt reservoir is primordial and not evolutionary.     

Voyager 1 exited the solar wind at a distance of approximately 85 Au from the Sun

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow. Theory predicts that a termination shock marks this boundary, with locations ranging from a few to over 100 au (1 Au approximately 1.5 x 10(8) km, the distance from Earth to the Sun). 'Pick-up ions' that originate as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays. Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of approximately 85 Au (heliolatitude approximately 34 degrees N), then re-entered the supersonic solar wind about 200 days later at approximately 87 au from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.     

Domination of heliosheath pressure by shock-accelerated pickup ions from observations of neutral atoms

The solar wind blows an immense magnetic bubble, the heliosphere, in the local interstellar medium (mostly neutral gas) flowing by the Sun. Recent measurements by Voyager 2 across the termination shock, where the solar wind is slowed to subsonic speeds before entering the heliosheath, found that the shocked solar wind plasma contains only approximately 20 per cent of the energy released by the termination shock, whereas energetic particles above approximately 28 keV contain only approximately 10 per cent; approximately 70 per cent of the energy is unaccounted for, leading to speculation that the unmeasured pickup ions or energetic particles below 28 keV contain the missing energy. Here we report the detection and mapping of heliosheath energetic ( approximately 4-20 keV) neutral atoms produced by charge exchange of suprathermal ions with interstellar neutral atoms. The energetic neutral atoms come from a source approximately 60 degrees wide in longitude straddling the direction of the local interstellar medium. Their energy spectra resemble those of solar wind pickup ions, but with a knee at approximately 11 keV instead of approximately 4 keV, indicating that their parent ions are pickup ions energized by the termination shock. These termination-shock-energized pickup ions contain the missing approximately 70 per cent of the energy dissipated in the termination shock, and they dominate the pressure in the heliosheath.     

太阳风离子在月面的反射过程及对月面电场的影响

为探究月表磁异常区对太阳风离子产生反射的原理, 基于嫦娥二号卫星携带太阳风离子探测器的探测数据, 用单粒子模拟法反演太阳风离子运动, 并分析离子入射角和反射角的分布. 实验结果表明： 太阳风离子先被月壤向各方向大范围散射, 再被月面电场向天顶方向加速; 太阳风中, 月面有45~75 V电势, 该正电势对月面反射的太阳风离子有显著影响.     

Stabilization of the disk around beta Pictoris by extremely carbon-rich gas

The edge-on disk surrounding the nearby young star beta Pictoris is the archetype of 'debris disks', which are composed of dust and gas produced by collisions between--and evaporation of--planetesimals, analogues of Solar System comets and asteroids. These disks may provide insight into the formation and early evolution of terrestrial planets. Previous work on beta Pic concluded that the disk gas has roughly solar abundances of elements, but this poses a problem because such gas should rapidly be blown away from the star, contrary to observations showing a stable gas disk in keplerian rotation. Here we report the detection of singly and doubly ionized carbon (C II, C III) and neutral atomic oxygen (O I) gas in the beta Pic disk. Carbon is extremely overabundant relative to every other measured element. This appears to solve the problem of the stable gas disk, because the carbon overabundance should keep the gas disk in keplerian rotation. The overabundance may indicate that the gas is produced from material more carbon-rich than expected of Solar System analogues.     

A collision in 2009 as the origin of the debris trail of asteroid P/2010 A2

The peculiar object P/2010?A2 was discovered in January 2010 and given a cometary designation because of the presence of a trail of material, although there was no central condensation or coma. The appearance of this object, in an asteroidal orbit (small eccentricity and inclination) in the inner main asteroid belt attracted attention as a potential new member of the recently recognized class of main-belt comets. If confirmed, this new object would expand the range in heliocentric distance over which main-belt comets are found. Here we report observations of P/2010?A2 by the Rosetta spacecraft. We conclude that the trail arose from a single event, rather than a period of cometary activity, in agreement with independent results. The trail is made up of relatively large particles of millimetre to centimetre size that remain close to the parent asteroid. The shape of the trail can be explained by an initial impact ejecting large clumps of debris that disintegrated and dispersed almost immediately. We determine that this was an asteroid collision that occurred around 10 February 2009.     

变质量少体问题及其应用

该文以变质量质点动力学方程（密歇尔斯基方程）为基础,建立了变质量少体问题的运动方程,并利用小参数方法得到了变质量少体问题的分析解。文中还将研究结果应用到彗星运动上,讨论彗星轨道中非引力效应。指出非引力效应会导致某些彗星轨道半长径和偏心率等产生长期和周期变化,这些变化将明显地影响彗星运动的周期以及过近日点的时间和地点。因此在这些彗星精密定轨和探讨其轨道动力演化时应当考虑这种非引力效应。文末还具体计算了恩克彗星轨道中非引力效应。     

Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock

The solar wind blows outward from the Sun and forms a bubble of solar material in the interstellar medium. The termination shock occurs where the solar wind changes from being supersonic (with respect to the surrounding interstellar medium) to being subsonic. The shock was crossed by Voyager 1 at a heliocentric radius of 94 au (1 au is the Earth-Sun distance) in December 2004 (refs 1-3). The Voyager 2 plasma experiment observed a decrease in solar wind speed commencing on about 9 June 2007, which culminated in several crossings of the termination shock between 30 August and 1 September 2007 (refs 4-7). Since then, Voyager 2 has remained in the heliosheath, the region of shocked solar wind. Here we report observations of plasma at and near the termination shock and in the heliosheath. The heliosphere is asymmetric, pushed inward in the Voyager 2 direction relative to the Voyager 1 direction. The termination shock is a weak, quasi-perpendicular shock that heats the thermal plasma very little. An unexpected finding is that the flow is still supersonic with respect to the thermal ions downstream of the termination shock. Most of the solar wind energy is transferred to the pickup ions or other energetic particles both upstream of and at the termination shock.     

Carbon compounds in Apollo 11 lunar samples

Are they entrapped solar wind particles, meteoritic or cometary contributions, remnants of a primordial atmosphere or products of the degassing of the Moon ?     

太阳风起源区等离子体动力学机制

 根据太阳风的起源,对太阳风的等离子体动力学理论进行研究。对磁流体力学（MHD）做了详细概述,在此基础上讨论稳定态下的震波结构,并进行详细的公式推导,特别是对它的不连续结构与旋转不连续的性质进行说明。通过对卫星观测结果的分析,得到两种磁场方向不连续面：切向不连续（TD）和旋转不连续（RD）。研究表明,TD的特性为垂直不连续面的磁场分量为零,电离子不通过该面;RD两侧的磁场方向不同但是大小相同,且有电离子通过该不连续面。太阳风中旋转不连续面主要与高速太阳风有关。通过分析6种不连续结构得到：在通过界面的磁通量保持不变的情况下,切向磁场会任意改变方向,出现旋转不连续现象,且此现象能够通过一维空间结构图方法加以验证。通过对比推导结果与数据验证图,发现数据图的分析支持公式的推导,证实太阳风起源区等离子存在旋转不连续机制。     

Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind

Energy is required to heat the outer solar atmosphere to millions of degrees (refs 1, 2) and to accelerate the solar wind to hundreds of kilometres per second (refs 2-6). Alfvén waves (travelling oscillations of ions and magnetic field) have been invoked as a possible mechanism to transport magneto-convective energy upwards along the Sun's magnetic field lines into the corona. Previous observations of Alfvénic waves in the corona revealed amplitudes far too small (0.5?km?s(-1)) to supply the energy flux (100-200?W?m(-2)) required to drive the fast solar wind or balance the radiative losses of the quiet corona. Here we report observations of the transition region (between the chromosphere and the corona) and of the corona that reveal how Alfvénic motions permeate the dynamic and finely structured outer solar atmosphere. The ubiquitous outward-propagating Alfvénic motions observed have amplitudes of the order of 20?km?s(-1) and periods of the order of 100-500?s throughout the quiescent atmosphere (compatible with recent investigations), and are energetic enough to accelerate the fast solar wind and heat the quiet corona.     

A late Miocene dust shower from the break-up of an asteroid in the main belt

Throughout the history of the Solar System, Earth has been bombarded by interplanetary dust particles (IDPs), which are asteroid and comet fragments of diameter approximately 1-1,000 microm. The IDP flux is believed to be in quasi-steady state: particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution, dynamical ejection, and planetary or solar impact. Because IDPs are rich in 3He, seafloor sediment 3He concentrations provide a unique means of probing the major events that have affected the IDP flux and its source bodies over geological timescales. Here we report that collisional disruption of the >150-km-diameter asteroid that created the Veritas family 8.3 +/- 0.5 Myr ago also produced a transient increase in the flux of interplanetary dust-derived 3He. The increase began at 8.2 +/- 0.1 Myr ago, reached a maximum of approximately 4 times pre-event levels, and dissipated over approximately 1.5 Myr. The terrestrial IDP accretion rate was overwhelmingly dominated by Veritas family fragments during the late Miocene. No other event of this magnitude over the past approximately 10(8) yr has been deduced from main belt asteroid orbits. One remarkably similar event is present in the 3He record 35 Myr ago, but its origin by comet shower or asteroid collision remains uncertain.     

Mediation of the solar wind termination shock by non-thermal ions

Broad regions on both sides of the solar wind termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the solar wind have been measured by the spacecraft Voyager 1 (refs 1-5) and Voyager 2 (refs 3, 6). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3-5). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August-1 September 2007 (refs 7-9). Unlike Voyager 1, Voyager 2 is making plasma measurements. Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the termination shock structure and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident solar wind.     

Possible cometary origin of heavy noble gases in the atmospheres of Venus, Earth and Mars

Models that trace the origin of noble gases in the atmospheres of the terrestrial planets (Venus, Earth and Mars) to the 'planetary component' in chondritic meteorites confront several problems. The 'missing' xenon in the atmospheres of Mars and Earth is one of the most obvious; this gas is not hidden or trapped in surface materials. On Venus, the absolute abundances of neon and argon per gram of rock are higher even than those in carbonaceous chondrites, whereas the relative abundances of argon and krypton are closer to solar than to chondritic values (there is only an upper limit on xenon). Pepin has developed a model that emphasizes hydrodynamic escape of early, massive hydrogen atmospheres to explain the abundances and isotope ratios of noble gases on all three planets. We have previously suggested that the unusual abundances of heavy noble gases on Venus might be explained by the impact of a low-temperature comet. Further consideration of the probable history of the martian atmosphere, the noble-gas data from the (Mars-derived) SNC meteorites and laboratory experiments on the trapping of noble gases in ice lead us to propose here that the noble gases in the atmospheres of all of the terrestrial planets are dominated by a mixture of an internal component and contribution from impacting icy planetesimals (comets). If true, this hypothesis illustrates the importance of impacts in determining the volatile inventories of these planets.